It is known that the composition of the exhaust gases produced in controlled ignition engines (for instance in petrol or gas engines in which the combustion of the air/fuel mixture is triggered, following command by the control system of the engine, by the ignition of a spark at a predetermined moment), depends, among other things, on the composition of the air/fuel mixture that is injected into the cylinders. These engines can in particular operate using a lean fuel mixture, i.e. having a ratio (A/F) greater than the stoichiometric ratio (A/F).sub.ST, or, in an equivalent manner, having a titre .lambda., defined by the ratio (A/F)/(A/F).sub.ST, greater than 1. In these circumstances, the exhaust gases form a highly oxidising atmosphere as a result of which a normal three-way catalyst (TWC) is not sufficient to remove the nitrogen oxide component NOx produced during combustion. As shown in FIG. 1, the efficiency of removal of nitrogen oxides .eta..sub.NOx for a normal three-way catalyst is very high and close to 1 when the engine operates with a rich air/fuel mixture (having a ratio (A/F) lower than the stoichiometric ratio (A/F).sub.ST or, in an equivalent manner, a titre .lambda. lower than 1), but deteriorates rapidly for values of the ratio (A/F) that are greater than the stoichiometric ratio (A/F).sub.ST. Vice versa, the efficiency of removal of carbon monoxide .eta..sub.co, and, respectively, of non-combusted hydrocarbons .eta..sub.HC is low in the presence of a rich air/fuel mixture and close to 1 for a lean air/fuel mixture.
A solution that is commonly used is to dispose, downstream of a three-way pre-catalyst, a main catalyst formed by a trap able to absorb and store the nitrogen oxides (a so-called NOx TRAP). When the trap is saturated, however, it is no longer able to perform this function and must therefore be emptied by means of a regeneration process which consists in creating, within the trap, an atmosphere such as to give rise to reduction reactions of the nitrogen oxides NOx. Molecular nitrogen N.sub.2, steam and other non-polluting products are released during these reactions. The reducing atmosphere is obtained by causing a mixture of exhaust gases composed chiefly of carbon monoxide CO and non-combusted hydrocarbons HC and substantially free from nitrogen oxides NOx to flow into the trap, as is the case when the engine operates with a rich air/fuel mixture. In this case, there is an overproduction of carbon monoxide CO and non-combusted hydrocarbons HC that the three-way catalyst is not able to remove as a result of the fact that it is not very efficient in the presence of a rich mixture, while the emissions of nitrogen oxides NOx are drastically reduced. The exhaust gas mixture thus produced reacts with the nitrogen oxides NOx present in the trap, thereby emptying it. During the regeneration process, moreover, the titre downstream of the trap is substantially stoichiometric.
The use of traps of the type described above raises a further problem connected with the fact that they also store sulphur oxides SOx. Even though the capture of sulphur oxides SOx is a slower process than the capture of nitrogen oxides NOx, provision must nevertheless also be made for desulphurisation cycles in order to maximise the available capacity and the efficiency of the trap.
Moreover, in order to ensure that the trap is highly efficient and to limit the consumption of fuel and polluting emissions, these regenerations and desulphurisations must be carried out according to well defined strategies.
The control systems available at present are based on units provided with a first oxygen sensor (LAMBDA sensor of linear type) disposed upstream of the catalyst TWC and a second oxygen sensor (LAMBDA sensor of on/off type) disposed downstream of the trap. The regeneration strategies currently used estimate the degree of filling of the trap solely from mapping of the engine and from physical and mathematical models, to whose parameters predetermined values are assigned at the calibration stage. The efficiency of control depends, among other things, on the accuracy of these values which cannot, however, subsequently be automatically updated during the operation of the system.
The systems described above are disadvantageous as they are not able to take account of any deviations with respect to nominal operating conditions. In particular, the performances of the various components are not constant over time, but show drifts due, for instance, to ageing or to the onset of malfunctions, as a result of which the values of the parameters of the physical and mathematical models set during calibration are not longer adapted correctly to describe the state of the system. In these circumstances, therefore, conventional regeneration strategies do not guarantee that measures to reset the efficiency of the trap are carried out when they are actually necessary. Consequently, it may be case that the trap remains saturated for longer than it should before it is emptied, with a substantial increase in polluting emissions from the vehicle. Moreover, the duration of the regenerations is also predetermined and cannot be modified if it proves to be inadequate.